Power conversion techniques using pulse width modulation (PWM) controllers typically vary the duty cycle, the phase, or the frequency of the PWM signal to control the flow of power in a power supply. Analog or digital PWM controllers may be used for PWM signal generation. Regardless of the PWM signal generation technique used, the general goal is to provide as much resolution as possible to minimize the output voltage ripple in the power supply.
For analog PWM signal generation, it is difficult to generate stable and consistent frequency PWM signals over temperature, process and when subjected to a range of ambient noise conditions.
The digital generation of PWM signals with high frequency resolution is very difficult. PWM signal generators typically use a digital counter with an associated comparator and PWM period register. The frequency resolution of this typical PWM circuit is determined by the frequency of the clock signal that drives the counter. To obtain one (1) nanosecond of frequency resolution, the counter must be clocked with a 1 GHz (Billion Hertz) signal. In many technologies, this high a frequency of operation is impossible to obtain, the required power consumption is too high, and/or the cost is prohibitive.
Most common power conversion controllers are implemented with analog circuitry that do not require high frequency clocks to achieve frequency resolution. It is know in the art of power supply design to increase the clock rate of the PWM generator counter to improve frequency resolution. This approach works well at lower frequencies for applications such as motor control but becomes impractical for higher frequency power conversion circuits. Very few companies have attempted to provide microcontrollers and/or digital signal processors with high resolution digital PWM generators to serve the power conversion market. Historically, the PWM parameter that was considered important was the duty cycle (PWM pulse width). Only recently, have “resonant mode” power conversion circuits become “fashionable” because of the desire to improve the power conversion efficiency. Resonant mode power conversion uses the PWM frequency to control the flow of power, instead of the PWM duty cycle.